3GPP Long Term Evolution (LTE) technology is a mobile broadband wireless communication technology in which transmissions from base stations (referred to as eNBs) to mobile stations (referred to as user equipment (UE)) are sent using orthogonal frequency division multiplexing (OFDM). OFDM splits the signal into multiple parallel subcarriers in frequency. The basic unit of transmission in LTE is a resource block (RB) which in its most common configuration consists of 12 subcarriers and 7 OFDM symbols (one slot). A unit of one subcarrier and 1 OFDM symbol is referred to as a resource element (RE) see FIG. 1. Thus, an RB consists of 84 REs. An LTE radio subframe is composed of multiple resource blocks in frequency with the number of RBs determining the bandwidth of the system and two slots in time, see FIG. 2. Furthermore, the two RBs in a subframe that are adjacent in time are denoted as an RB pair.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms.
The signal transmitted by the eNB in a downlink (the link carrying transmissions from the eNB to the UE) subframe may be transmitted from multiple antennas and the signal may be received at a UE that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. In order to demodulate any transmissions on the downlink, a UE relies on reference signals (RS) that are transmitted on the downlink. The RS consists of a collection of reference symbols and these reference symbols and their position in the time-frequency grid are known to the UE and hence can be used to determine channel estimates by measuring the effect of the radio channel on these symbols.
It should be noted in this context that the channel a UE measures is not necessarily from a particular physical transmit antenna element at the eNB to the UEs receiver antenna element, since the UE bases the measurement on a transmitted RS and the channel it measures depends on how the particular RS is transmitted from the multiple physical antenna elements at the eNB. Therefore, the concept of an antenna port is introduced, where an antenna port is a virtual antenna that is associated with an RS.
In 3GPP TS 36.211, an antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed. This definition also applies to the present disclosure.
A UE measures the channel from an antenna port to the receiver antenna element using the RS associated with that antenna port. Which physical transmit antenna element, or group of elements that are actually used for the transmission of this RS is transparent and also irrelevant for the UE; the transmission on an antenna port may use a single physical antenna element or a combination of signals from multiple antenna elements. Hence, the precoding or mapping to physical antenna elements that was applied by the eNB is transparently included in the effective channel that the UE measures from the antenna port.
An example of utilization of multiple antenna elements is the use of transmit precoding to direct the transmitted energy towards one particular receiving UE, by using all available antenna elements for transmission to transmit the same message, but where individual phase and possibly amplitude weights are applied at each transmit antenna element. This is sometimes denoted UE-specific precoding and the RS in this case is denoted UE-specific RS. If the UE-specific RS in the RB is precoded with the same UE-specific precoding as the data, then the transmission is performed using a single virtual antenna, i.e. a single antenna port, and the UE need only to perform channel estimation using this single UE-specific RS and use it as a reference for demodulating the data in this RB. In other words, the UE does not need to know the precoding vector that was applied by the eNB when transmitting the data. Selecting and adapting the precoding vector is typically left to the implementation, and is thus not described in standard specifications.
The UE-specific RS are transmitted only when data is transmitted to a UE in the subframe, otherwise they are not present. In LTE, UE-specific RS are included as part of the RBs that are allocated to a UE for reception of user data. Examples of UE-specific reference signals in LTE can be found in FIG. 3 where for example all RE denoted R7 contains modulated reference symbols belonging to one “RS”. Hence, what is known as an RS is a collection reference symbols transmitted in a set of distributed REs.
Another type of reference signals are those that can be used by all UEs and thus have wide cell area coverage. One example of these is the common reference signals (CRS) that are used by UEs for various purposes including channel estimation and mobility measurements. These CRS are defined so that they occupy certain pre-defined REs within all the subframes in the system bandwidth irrespectively of whether there is any data being sent to users in a subframe or not. These CRS are shown as “reference symbols” in FIG. 2.
Messages transmitted over the radio link to users can be broadly classified as control messages or data messages. Control messages are used to facilitate the proper operation of the system as well as proper operation of each UE within the system. Control messages could include commands to control functions such as the transmitted power from a UE, signaling of RBs within which the data is to be received by the UE or transmitted from the UE and so on. Examples of control messages are the physical downlink control channel (PDCCH) which for example carry scheduling information and power control messages, the physical HARQ indicator channel (PHICH), which carries ACK/NACK in response to a previous uplink transmission and the physical broadcast channel (PBCH) which carries system information.
In LTE Rel-10, control messages are demodulated using the CRS (except for the R-PDCCH, see below), hence they have a wide cell coverage to reach all UEs in the cell without having knowledge about their position. The first one to four OFDM symbols, depending on the configuration, in a subframe are reserved for control information, see FIG. 2. Control messages could be categorized into those types of messages that need to be sent only to one UE (UE-specific control) and those that need to be sent to all UEs or some subset of UEs numbering more than one (common control) within the cell being covered by the eNB
It shall be noted in this context that in future LTE releases, there will be new carrier types which may not have a PDCCH transmission or transmission of CRS.
Control messages of PDCCH type are transmitted in multiples of units called control channel elements (CCEs) where the modulated symbols of each CCE maps to 36 REs. A PDCCH may have an aggregation level (AL) of 1, 2, 4 or 8 CCEs to allow for link adaptation of the control message. Thus, the term “aggregation level” is used in this disclosure to refer to the number of CCEs that form a PDCCH. Furthermore, the 36 modulated symbols of each CCE is mapped to 9 resource element groups (REG) consisting of 4 RE each. These REG are distributed over the whole bandwidth to provide frequency diversity for a CCE, see FIG. 4. Hence, a PDCCH, which consists of up to 8 CCEs, spans the entire system bandwidth in the first n={1, 2, 3 or 4} OFDM symbols, depending on the configuration value of n.
After channel coding, scrambling, modulation and interleaving of the control information, the modulated symbols are mapped to the resource elements in the control region. In total there are NCCE CCEs available for all the PDCCH to be transmitted in the subframe and the number NCCE varies from subframe to subframe depending on the number of control symbols n, the number of antenna ports associated with CRS and the configured number of HARQ indicator channels (PHICH).
As the number of control symbols n is indicated by the control format indicator channel (PCFICH) in every subframe, the value of NCCE varies from subframe to subframe, the terminal needs to blindly determine the position and the number of CCEs used for its PDCCH. Also, a UE need to blindly search and detect if the control channel is valid for it, without knowing the CCE aggregation level beforehand, which can be a computationally intensive decoding task due to the large value of NCCE. Therefore, some restrictions have been introduced in the number of possible blind decodings a terminal needs to go through. For instance, the CCEs are numbered and CCE aggregation levels of size K can only start on CCE numbers evenly divisible by K, see FIG. 5.
The set of CCEs where a terminal needs to blindly decode and search for a valid PDCCH is called a search space. This is the set of CCEs on a AL a terminal should monitor for scheduling assignments or other control information, see example in FIG. 6. In each subframe and on each AL, a terminal will attempt to decode all the PDCCHs that can be formed from the CCEs in its search space. If the CRC checks, then the content of the PDCCH is assumed to be valid for the terminal and it further processes the received information. Often, two or more terminals will have overlapping search spaces and the network has to select one of them for scheduling and transmission of the control channel. When this happens, the non-scheduled terminals are said to be blocked. The search spaces vary pseudo-randomly from subframe to subframe to minimize the probability of blocking.
A search space is further divided to a common and a terminal (UE)-specific part. In the common search space, the PDCCH containing information to all or a group of terminals is transmitted (paging, system information etc). If carrier aggregation is used, a terminal will find the common search space present on the primary component carrier (PCC) only. The common search space is restricted to aggregation levels 4 and 8 to give sufficient channel code protection for all terminals in the cell. Since it is a broadcast channel, link adaptation can not be used. The m8 and m4 first PDCCH (with lowest CCE number) in an AL of 8 or 4 respectively belong to the common search space. For efficient use of the CCEs in the system, the remaining search space is terminal specific at each aggregation level.
A CCE consists of 36 QPSK modulated symbols that map to the 36 RE unique for this CCE. To maximize the diversity and interference randomization, interleaving of all the CCEs is used before a cell specific cyclic shift and mapping to REs, see the processing steps in FIG. 7. Note that in most cases some CCEs are empty during transmission due to the PDCCH location restriction to terminal search spaces and aggregation levels. The empty CCEs are included in same the interleaving process and mapping to RE as any other PDCCH to maintain the search space structure. Empty CCE are set to zero power and this power can instead be used by non-empty CCEs to further enhance the link performance of the PDCCH transmission.
Furthermore, to enable the use of 4 antenna TX diversity, a group of 4 adjacent QPSK symbols in a CCE is mapped to 4 adjacent RE, denoted a RE group (REG). Hence, the CCE interleaving is quadruplex (group of 4) based and the mapping process has a granularity of 1 REG and one CCE corresponds to 9 REGs (=36 RE).
Transmission of the physical downlink shared data channel (PDSCH) to UEs, is using the RE in a RB pair that are not used for control messages or RS. The PDSCH can either be transmitted using the UE-specific reference symbols or the CRS as a demodulation reference, depending on the configured transmission mode. The use of UE-specific RS allows a multi-antenna eNB to optimize the transmission using pre-coding of both data and reference signals being transmitted from the multiple antennas so that the received signal energy increases at the UE. Consequently, the channel estimation performance is improved and the data rate of the transmission may be increased.
In Rel-10 of LTE a relay control channel was also defined, denoted R-PDCCH, for transmitting control information from eNB to relay nodes. The R-PDCCH is placed in the data region, hence, similar to a PDSCH transmission. The transmission of the R-PDCCH can either be configured to use CRS to provide wide cell coverage, or relay node (RN) specific reference signals to improve the link performance towards a particular RN by precoding, similar to the enhancement of the PDSCH transmission with UE-specific RS. The UE-specific RS is in the latter case used also for the R-PDCCH transmission. The R-PDCCH occupies a number of configured RB pairs in the system bandwidth and is thus frequency multiplexed with the PDSCH transmissions in the remaining RB pairs, see FIG. 8.
In LTE Rel.11 discussions, attention has turned to adopting the same principle of UE-specific transmission as for the PDSCH and the R-PDCCH also for control channels (including PDCCH, PHICH, PCFICH, and PBCH) by allowing the transmission of generic control messages to a UE using such transmissions to be based on UE-specific reference signals. This means that precoding gains can be achieved also for the control channels, thereby achieving an extended or enhanced control channel. Another benefit is that different RB pairs configured for the extended control channel can be configured in different cells or different transmission points within a cell. Thereby, intercell interference coordination between extended control channels may be achieved. This frequency coordination is not possible with the PDCCH since the PDCCH spans the whole bandwidth. FIG. 9 shows an extended or enhanced PDCCH (ePDCCH) which, similarly to the CCE in the PDCCH, is divided into multiple groups and mapped to one of the RB pairs configured for enhanced control channels, here denoted enhanced control regions.
Note that in FIG. 9, the enhanced control region does not start at OFDM symbol zero, to accommodate simultaneous transmission of a PDCCH in the subframe. However, as mentioned above, there may be carrier types in future LTE releases that do not have a PDCCH, in which case the enhanced control region could start from OFDM symbol zero within the subframe.
Even if the enhanced control channel enables UE-specific precoding and also possibly localized transmission (within one RB pair) as illustrated in FIG. 9, it may in some cases be useful to be able to transmit an enhanced control channel in a broadcasted, wide-area coverage fashion. This is useful if the eNB does not have reliable information to perform precoding towards a certain UE. In this case a wide-area coverage transmission is more robust, although the precoding gain is lost. Another case when broadcast and wide-area transmission is useful is when the particular control message is intended for more than one UE. In this case, UE-specific precoding cannot be used. An example of this is the transmission of common control information using ePDCCH (i.e. in the common search space). Yet another case where wideband transmission is useful is when subband precoding is utilized. Since the UE estimates the channel in each RB pair individually, the eNB can choose different precoding vectors in the different RB pairs, if the eNB has such information that the preferred precoding vectors are different in different parts of the frequency band. In any of these cases, a distributed transmission may be used, see FIG. 10, where the eREG belonging to the same ePDCCH are distributed over the enhanced control regions.
Thus, there is a need for mechanisms for providing both localized and distributed transmission of downlink control information in an efficient and flexible way.